gmail pop access suddenly failing?

My FOSS Zimbra account is configured to pull in email from a couple of gmail accounts via pop3. This has been working well for well over a year. Suddenly, sometime around March 1st it stopped working. I had not made any config changes on either the Zimbra or gmail side. The account config seems fine, the "test" button works. The mailbox.log files show successful connections to the gmail pop server, "importing data" and "import completed" mentries, but no messages are retrieved. There are several unread messages on each account, so I'm at a loss to understand what might be wrong.